boxy: replace Kodi with Plasma Bigscreen

Add plasma-bigscreen role with package built from upstream master
(not yet in nixpkgs, tracking NixOS/nixpkgs#428077).

Changes:
- New role: roles/plasma-bigscreen/ (module + package derivation)
- boxy configuration: swap roles.kodi for roles.plasma-bigscreen
- Keeps all existing functionality: Jellyfin, Stremio, Firefox,
  KDE Connect, app-launcher-server, AVR volume control (kodi user)
- Autologins to plasma-bigscreen-wayland session instead of plasma

NOTE: First build will fail with a hash mismatch on the source
fetch — copy the correct sha256 from the error into package.nix.
Some dep attribute names may also need adjustment on first build.

flake.nix

@@ -185,6 +185,7 @@
         ./machines/boxy/configuration.nix
         {
           home-manager.users.johno = import ./home/home-media-center.nix;
+          # kodi user: AVR volume control + minimal Plasma config for Bigscreen session
           home-manager.users.kodi = import ./home/home-kodi.nix;
           home-manager.extraSpecialArgs = { inherit system; };
         }